Can't see the alpha/texture of sculpting brush when changing size
You need to enable the Falloff visibility in the Tool settings. You can access it in the header of the 3D view while sculpting or in the Tools properties in the properties editor.

Arguably the tooltip and the panel in which you access the overlay properties are misleading.
You can also change the opacity there. Enabling the Brush icon button will remove the overlay once you have clicked so it doesn't cover your mesh while sculpting.